1. Field of the Invention.
The invention relates to a method for manufacturing a web of paper or paperboard using a spray-coating technique wherein the web treatment substance such as a coating is applied to the surface of a moving web by means of spraying the substance from high-pressure spray nozzles onto the web.
The invention also relates to a paper grade manufactured using the method.
2. Description of the Related Art.
Currently, application of coating to a web of paper and paperboard is carried out using a plurality of different methods. Each application method has certain characteristic features that affect the quality of the finished product and its manufacturing process. In regard to the optimal production process, the choice must be made based on the actual running speed and strength of the web. The qualities to be rendered to the finished product are determined by the end use of the product, which in practice is defined by the printing method applied to the product and the intended degree of quality to be rendered to the final printed surface. In general, good printability can be achieved by giving the web a sufficiently smooth and even surface having an extremely high brightness. These qualities can be attained by applying a sufficient number of coating layers to the web being processed and then calendering the same in the different phases of paper manufacture. Obviously, as a high number of treatment steps increase the end price of the product, it must be understood that the choice of the proper manufacturing technique is ultimately dictated by the qualities that are desired from the end product. After the suitable process for the desired end product is chosen, it is possible to select such a finishing technique that renders the optimum result in terms of production efficiency and end product quality.
One promising application method is the spray-coating technique, wherein the coating finish or other web treatment material is applied to the surface of a paper or paperboard web by means of high-pressure spray nozzles. In this method, the web treatment material is pressurized to a high pressure and then sprayed from a small-orifice spray nozzle at a high velocity onto the web. A benefit of the method is that it imposes minimal stress on the web resulting in a good runnability behavior of the spray-coating apparatuses. As it is implemented in a fully noncontacting fashion, this kind of application excels over conventional blade and film-transfer techniques by being more gentle and less stressing to the web thus facilitating a production efficiency increase of about 5% as compared with blade application. In a different point of view, this allows the web to be produced from a stock of weaker strength, which is a particular benefit in making paper from recycle fiber. Contacting application methods such as blade coating or film-transfer coating always need between the web surface and the excess-coating-doctoring element a gap that determines the thickness of the applied coat. Due to this and other factors, the final coat weight is affected by quality variations in the base sheet, among other things by its profile deviations and surface roughness. In contrast, a spray-coater applies to the web under all circumstances a coating layer of constant weight entirely unaffected by base sheet quality variations. Moreover, spray-coating can be performed to a base sheet whose moisture content is higher and, hence, whose strength is lower than what has been acceptable earlier.
The price and quantity of raw materials used in papermaking have a crucial impact on the competitiveness of paper grades produced. Herein, spray-coating allows the production of the base sheet from a cheaper stock of lower strength without essentially compromising the end product quality. Inasmuch this coating method is free from streaking that typically is a handicap of other methods by occurring during coat smoothing and metering, it also permits the use of filler-grade calcium carbonate that is cheaper than coating-grade calcium carbonate. In the spray-coating method, the penetration of web treatment substance into the base web is minimal thus leaving the pigment or size particles onto the web surface. Resultingly, web porosity can be reduced by surface sizing resulting in reduced penetration of coating agent into the web, whereby the covering effect of surface size increases.
Hence, spray-coating method can be used for producing novel grades of paper.